[roll third_party/pigweed] pw_thread_embos: add thread creation for embOS

Adds thread creation support to embOS.

Also updates existing documentation to reflect that this is now
supported.

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/44144

third_party/pigweed Rolled-Commits: f4789286cc5662c..6e5d43e5d97b9f2
Roller-URL: https://ci.chromium.org/b/8847224077525778864
Cq-Cl-Tag: roller-builder:pigweed-experimental-roller
Cq-Cl-Tag: roller-bid:8847224077525778864
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: I2708caf31f8575605ac05be2d1b8af0a2595fe19
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/experimental/+/45403
Reviewed-by: Pigweed Integration Roller <pigweed-integration-roller@pigweed.google.com.iam.gserviceaccount.com>
Commit-Queue: Pigweed Integration Roller <pigweed-integration-roller@pigweed.google.com.iam.gserviceaccount.com>
diff --git a/third_party/pigweed b/third_party/pigweed
index f478928..6e5d43e 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit f4789286cc5662ce44a7684b3f9e7974bd41643c
+Subproject commit 6e5d43e5d97b9f25c33190972288f4fc46942880